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  • Hello. This is 6 Minute English from

  • BBC Learning English. I'm Neil.

  • And I'm Sam.

  • Over the past 18 months, we've heard

  • a lot about the human immune

  • system - the cells in our bodies

  • that fight diseases like coronavirus.

  • We know that in humans the blood

  • stream carries immune cells

  • around our body.

  • But what about trees and plants?

  • They don't have blood, so how

  • do they protect themselves?

  • That's a good question, Sam, and

  • the answer involves memory. Us,

  • humans, store memories in our brain,

  • but our body also remembers things,

  • including stressful situations

  • from the past, which it stores

  • in our genes. The information

  • gets passed on to our

  • children genetically.

  • But surely trees don't have

  • memories, Neil! I mean, do

  • you think a tree can remember

  • being young or what it

  • was doing last year?

  • Well, not exactly, but trees

  • grow rings - a layer of wood

  • for each year of growth.

  • That could be a kind of memory.

  • In this programme, we'll be

  • asking whether trees can

  • remember - and if so, does

  • it make them stronger and

  • better able to fight disease?

  • But before that I have a

  • question for you, Sam. As I

  • said, trees grow a new ring

  • every year and by counting

  • them we can estimate their

  • age. One of Earth's longest

  • living trees is The Great

  • Bristlecone Pine, found on

  • the west coast of America.

  • But how long can these

  • trees live? Is it:

  • a) over 1,000 years?

  • b) over 3,000 years? or

  • c) over 5,000 years?

  • Wow, it'd be a job to count

  • the rings on those trees! I'll

  • say b) over 3,000 years.

  • OK, Sam, we'll reveal

  • the correct answer later.

  • Unlike us, trees don't have

  • blood and bones to protect them

  • from outside attacks, so how

  • exactly does a tree's

  • immune system work?

  • That's what BBC World Service

  • programme, CrowdScience, asked

  • bioscientist, Jurriaan Ton.

  • Here's what he said:

  • Plants in particular need to

  • have a very efficient immune

  • system for two important reasons.

  • Firstly, they sit at the bottom

  • of the food chain so there

  • are a lot of opportunistic

  • organisms out there, including

  • insect herbivores and microbial

  • pathogens who want to tap

  • into that biochemical energy

  • that is stored in plants.

  • The other reason is plants

  • are rooted to the ground -

  • they cannot escape from the

  • stressful conditions in

  • their environment.

  • It's hard for trees to protect

  • themselves. Unlike animals, they

  • can't run away, and they're

  • at the bottom of the food

  • chain - the plants and animals

  • linked in a chain of eating

  • weaker things and then being

  • eaten by stronger ones.

  • Rabbits eat grass and,

  • in turn, are eaten by foxes.

  • Right. If you are at the

  • bottom of the food chain,

  • everything wants to eat you,

  • including opportunistic animals.

  • If something is opportunistic,

  • it takes advantage of a

  • situation to gain some

  • benefit for itself. Tree

  • leaves are opportunities for

  • hungry insects and

  • caterpillars to eat.

  • So, trees need immunity

  • because they're under attack,

  • either from disease or from

  • living things wanting to

  • eat them. But what

  • about memory, Sam?

  • If trees can remember stress -

  • types of insects that eat it,

  • for example - they might be

  • better prepared in future.

  • For me, stress is a work

  • deadline or moving house,

  • but for trees it's more basic,

  • something like not

  • getting enough water.

  • Dr Estrella Luna-Diez believes

  • trees record stress in their

  • rings. A small ring, showing

  • that the tree didn't grow

  • much that year, indicates

  • some outside stress. She

  • explained more to BBC World

  • Service programme, CrowdScience:

  • Our hypothesis would be that,

  • depending on the level of

  • that stress - if it was a

  • really long-lasting drought

  • of a few years, then maybe

  • the tree can remember it for

  • a long time because it needs

  • to adapt to that hostile

  • environment. Now, maybe the

  • hypothesis would be the other

  • way around, maybe if it was

  • a very dry July for instance,

  • maybe the tree is not even

  • that bothered and then it

  • forgets within one year

  • because that memory of

  • stress is gonna be holding

  • it back on its growth,

  • for instance.

  • Dr Luna-Diez has a hypothesis -

  • an idea that explains how or

  • why something happens which

  • has yet to be tested to

  • see if it's correct.

  • Her hypothesis is that trees

  • remember stressful outside

  • events, something like a

  • drought - a long period of

  • time with little or no rain.

  • For a tree which has lived

  • for hundreds of years it

  • might be useful to remember

  • that 1947 was a

  • very dry summer.

  • On the other hand, maybe

  • that stressful year is

  • best forgotten. Maybe the

  • tree is not bothered - not

  • worried or concerned

  • because it's not

  • important to it.

  • So, trees do have memories -

  • but they don't let it

  • get them stressed!

  • Maybe that's the secret to

  • a long life! But what's

  • the answer to your

  • question, Neil?

  • Ah yes, I asked you how long

  • Earth's oldest trees, Great

  • Bristlecone Pines, can live.

  • I said b) over 3,000 years.

  • Was I right?

  • You were wrong, I'm

  • afraid, Sam. They live

  • even longer - over 5,000 years,

  • in fact all the way